Method for processing fibrous stalks

ABSTRACT

Method for processing crushed fibrous stalks containing pith, to separate the pith and the fiber, comprising gravity feeding fragments of the said stalks into the upper end of a vertical screening element, simultaneously feeding an aqueous carrier liquid either concurrently or separately in amounts sufficient to provide at least about 4.5 parts by weight of carrier liquid (e.g., water) per part by weight (dry weight basis) of fiber in the crushed fibrous stalks, centrifugally and helically propelling and gravity feeding the fragments through the treating zone defined by the screening element, in the absence of any extraneous artificially created air pressure differentials, so that a layer of axially aligned oriented fragments is formed on the inner surface of the screening element and the pith is separated by the rolling and rubbing action of the fragments on each other, and the separated pith particles which become relatively heavier due to absorption of carrier liquid are forced to the exterior of the screening element by centrifugal forces applied.

United States Patent Villavicencio 1 *Sept. 5, 1972 [72] Inventor: Eduardo Joel Brennan, Peru [73] Assignee: Process Evaluation and Development Corporation, New York, NY.

The portion of the term of this patent subsequent to Nov. 3, 1987, has been disclaimed.

[22] Filed: July 13, 1970 [21] Appl. No.: 54,580

' Related US. Application Data [63] Continuation-impart of Ser. No. 743,344, July 9, 1968, Pat. No. 3,537,142.

villavioencio,

[ Notice:

52 US. Cl ..19/7 511 m Cl. .nom 1/30 58 Field of Search ..19/90, 7, 8,26

[56] References Cited UNITED STATES PATENTS 3,537,142 11/1970 Villavicencio ..l9/26 2,729,858 1/1956 Horton et al. ..19/7

Primary Examiner-Dorsey Newton Attorney-William W. McDowell, Jr. and Kenneth E. Prince [57] ABSTRACT Method for processing crushed fibrous stalks containing pith, to separate the pith and the fiber, comprising gravity feeding fragments of the said stalks into the upper end of a vertical screening element, simultaneouslyfeeding an aqueous carrier liquid either concurrently or separately in amounts sufiicient to provide at least about 4.5 parts by weight of carrier liquid (e.g., water) per part by weight (dry weight basis) of fiber in the crushed fibrous stalks, centrifugally and helically propelling and gravity feeding the fragments through the treating zone defined by the screening element, in the absence of any extraneous artificially created air pressure differentials, so that a layer of axially aligned oriented fragments is formed on the inner surface of the screening element and the pith is separated by the rolling and rubbing action of the fragments on each other, and the separated pith particles which become relatively heavier due to absorption of carrier liquid are forced to the exterior of the screening element by centrifugal forces applied.

4Claims,2DrawingFigures PATENTEDSEP 5 m2 SHEEI 1 OF 2 METHOD FOR PROCESSING FIBROUS STALKS This application is a continuation-in-part of my earlier filed, copending and commonly assigned application Ser. No. 743,344 filed July 9, 1968 and now United States Pat. No. 3,5 37,142, granted Nov. 3, 1970 the entire disclosure of which is hereby incorporated herein by reference.

This invention relates to a method for processing fibrous vegetable materials so as to separate then into two portions, one of which is essentially 100 percent pith free and the other of which contains virtually all of the original pith.

The method of this invention separates fiber-containing stalk materials into fiber and pith fractions. The separated fractions can be used as desired. For example, the fiber portion can be used for pulp in the paper industry or as a basic raw material for making hardboard of various types. The pith fraction can be discarded to waste or dried and used as animal feed, chicken litter, animal bedding or burned as fuel in industrial or heating boilers. The method of this invention is especially suitable for obtaining substantially pith-free fiber from sugarcane bagasse for paper-making purposes, but its use is not restricted to sugarcane bagasse alone. The method is also suitable for processing other materials such as straw, flax, rice hulls, and similar vegetable matter.

The fibers of such materials are suitable for the production of pulp for use in paper or alpha-cellulose production, or other purposes, but their commercial use in such fields has been handicapped by the presence of varying amounts of pith and other nonfibrous material which is intimately admixed with the fibers, and which has little or no value in such pulps. Its separation from the fibers by most presently known methods is too costly to be practical.

Bagasse is the name given to the cellular material which forms the remains of sugarcane after the sugarcontaining juice has been extracted. In processing raw sugarcane, the cane stalks are first fed into a crushing roller and then into a series of roller type mills which squeeze the cane and force the sugar-containing juice from the broken cells for further processing and refining treatment. After substantially all of the sugar-containing juice has been expelled from the cane, the remainder, which is then called bagasse, consists of relatively long fibers of substantially pure cellulose together with a large amount of pith, which consists of broken cells and other materials, as well as 2 to 3 percent by weight of retained sugar. At this stage, the moisture content of the bagasse is relatively high, generally ranging between 48 and 52 percent by weight. l-leretofore, it has been customary to use this bagasse as a fuel for heating and refining the expressed juices, but this is relatively inefficient because of the high percentage of retained moisture in the bagasse. lt has been recognized that the long cellulosic fibers retained in bagasse have a high degree of potential utility for such purposes as paper pulp and the like, but the presence of the retained pith heretofore prevented the effective and economical utilization of the fiber because of the detrimental effect of the retained pith on the finished product. For example, if it is attempted to make paper from a pulp containing a large amount of retained pith, the paper is brittle and of extremely poor quality.

Heretofore, it has been proposed to treat the bagasse with further milling processes using hammer mills or disc mills in which the bagasse is subjected to a fluid treatment in an effort to provide a washing action to aid the mechanical process in separating the pith particles from the fibers. However, such processes have not been able to produce a finished fiber product of high enough purity at a low enough cost to allow its commercial use. In some of the prior art e.g., U.S. Pat. No. 2,650,176 to Horton et al, U.S. Pat. No. 2,744,037 to Lathrop and U.S. Pat. No. 3,011,220 to Keller et al.) recovery of sugar from bagasse is taught but no quantitative results are given.

In my above-identified prior application Ser. No. 743,344 now U.S. Pat. No. 3,537,142 I have described an improved apparatus and method for separating the pith from the fiber fraction of fibrous vegetable materials, especially sugarcane bagasse. When operating in accordance with the invention described in my said prior application one can consistently obtain from fresh bagasse (normally containing about 48 to 52 percent moisture) a fiber fraction (retained on 14 mesh) containing 95 to 97 weight percent fiber dry, soluble-free basis) without the necessity of using any washing fluids. This plus 14 mesh fiber fraction typically contains from 5 to 6 weight percent of water soluble materials.

I have now discovered that the use of certain critical minimum amounts of an aqueous carrier liquid in the method of my prior application permits the recovery of plus 14 mesh fiber fractions, also containing 95 to 97 weight percent fiber dry, soluble free basis) but substantially less, typically from 3 to 4 weight percent of water soluble material. If this discovery is applied to a secondary wet depithing of the fiber fraction from a dry depithing in accordance with my prior application, the resultant plus 14 mesh fiber recovered is essentially free of any pith whatsoever, i.e., contains at least about 98 or 99 weight percent fiber dry, soluble-free basis) and still has low water soluble content on the order of only 3 to 4 weight percent.

1 have also discovered that the new and improved process permits recovery of substantial amounts, of up to 50 55 percent or more, of the sugar retained in the bagasse after processing of the original cane in the sugar mill.

The invention will be further understood from the following more detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a side view, partially in section, of the apparatus described and claimed in my prior U.S. Pat. No. 3,5 37,142 modified for use in accordance with the present invention; and

FIG. 2 is a process flow diagram schematically illustrating the steps included in the presently preferred embodiment of the invention and certain other subsequent operations.

In summary the present invention provides a method for separating the fiber from the pith of crushed fibrous stalks, and where desired recovering substantial amounts of the sugar retained in bagasse, comprising gravity feeding fragments of the pith-containing crushed fibrous stalks into the upper end of a vertically disposed screening element; simultaneously feeding, separately or concurrently, an aqueous carrier liquid non-reactive with the fibrous component of said stalks; parting a helical flow path to the mixture of said fragments and carrier liquid and centrifugally forcing the fragments against the inner surface of the screening element by means of rotating agitating members to thereby form a layer of the fragments on the interior surface of the screening element, substantially all of which are oriented with their axes coaxial with the screening element axis; continuing to feed the said frag ments toward the lower end of the screening element by continuing contact with the rotating agitating members and because of the forces of gravity acting thereupon while separating the pith therefrom by the rolling and rubbing action of the fragments on each other; gradually increasing the working on said fragments as they progress from the upper inlet end to the lower exit end of the screening element to separate the pith from the fiber and permit the pith particles to absorb carrier liquid thus becoming substantially pith-free fibers from the lower exit end of the cylindrical screening element.

Any aqueous carrier liquid non-reactive with the fiber particles of the material being processed may be used. Suitable aqueous carrier liquids include fresh water, white water, black liquor, dilute aqueous sugar solution and the like. In view of its inherent non-reactive nature, ready availability and economy, water is the most preferred carrier liquid. Water or an aqueous sugar solution must be used in operations where sugar recovery from the bagasse is a desideratum. Where sugar recovery is not important, the use of black liquor is helpful in reducing chemicals requirements in later pulping operations on the fiber.

The carrier liquid must be used in amounts sufficient to provide a weight ratio of liquid to fiber of at least about 4.5 and preferably about 5.0, calculated on the dry weight of the fiber fraction in the material to be processed. There is no upper limit to the amount of carrier liquid that may be used. Practical considerations, e.g., desired output, liquid pumping capabilities, etc. will usually dictate a maximum liquid to fiber weight ratio of not more than about 20 or 25 to 1. In most circumstances it will be found that a weight ratio of from about 5:l to about :1 will provide optimum results.

Apparatus which may be used for the practice of the present invention has been disclosed in my prior application Ser. No. 743,344, now US. Pat. No. 3,537,142 the entire disclosure of which has been incorporated herein by reference. Such apparatus is illustrated in the accompanying FIG. 1 which is identical to FIG. 1 of my said prior patent except for the addition of a conduit 28 for the introduction of suitable amounts of aqueous carrier fluid into the feed chute 8. The amount of carrier fluid so introduced may be regulated, for example by valve 29. Other elements of the apparatus have already been generally described in my prior US. Pat. No. 3,537,142. Briefly, the reference number 1 denotes an outer casing having sidewalls 3 and 4, a top wall 2 having a feed opening 7 therein, and an inclined lower wall 5 leading to a pith outlet 21. The casing 1 forms a closed chamber 6 except for the feed opening 7, fiber outlet 24 and pith outlet 21. The horizontal cross section of casing 1 can be of any desired shape, i.e., circular, square, rectangular, or other polygonal arrangement. The only criterion is that there be sufficient space in the closed chamber 6 to accommodate the other elements of the apparatus. Mounted at any suitable place within the closed chamber 6 is a screening element 9 which is an essentially circular cylinder open at its upper end 22 and lower end23. The upper end 22 is snugly fitted within the top wall 2 of the casing l and is arranged so that the feed opening 7 in the top wall of the casing leads to the interior portion of the screening element 9 so that fibrous stalk material to be processed can be fed through conduit 8 through the opening 7 and into the treating zone defined by the screening element. In typical cases the screen will comprise two semicylindrical halves suitably bolted together at the ribs 26. In the presently preferred embodiment the screening element 9 is 36 inches high and has an inside diameter at its upper end 22 of about 38% inches gradually tapering to an inside diameter of about 38 inches at its lower end 23 and is perforated throughout with holes or openings 27. The latter may be from five thirty-second to five-sixteenth inch in diameter and sufficient in number to provide a free area of about 25 to 50 percent of the total inside surface area of screening element 9. Mounted inside of the screening element is a rotor assembly generally designated as 11. The rotor assembly is driven by connection to a main drive shaft 13 which is in turn powered by the motor 14 and a typical V-belt and pulley system generally designated as 15. The rotor assembly is maintained in axial alignment with the axis of the screening element 9 by means of bearing blocks 16 and 18. The rotor assembly includes a number of laterally extending hammers and other features of construction described in more detail in US. Pat. No. 3,537,142. In one presently preferred embodiment of the invention the screening element 9 is tapered slightly from its upper end 22 to its lower end 23 so that the clearance between the hammers at the upper end is slightly greater than the clearance between the hammers and the screen at the lower exit end of the device. This same end result can be accomplished, if desired, by varying the length of the hammers in the device. In the presently preferred embodiment, the taper is about one-fourth inch for each 10 inches of length of the screening element with a final clearance of approximately one-fourth inch between the screening element and the lowest hammers of the rotor assembly. This taper assures substantially uniform working of the material being processed as it passes downward through the treating zone defined by the screening element 9. The screening element may include a defibrating bar designated as 10 in FIG. 1 which, in conjunction with the hammers of the rotor assembly, will break up and chop into shorter lengths any overly long fibrous fragments passing through the treating zone. The defibrating bar extends all the way around the inner circumference of the screening element 9 and is secured thereto in any suitable manner, e.g., by welding. It is usually located in the upper onethird of the overall length of the screening element 9. A fiber recovery conduit 19 is connected to the lower end of the screening element 23 and has inclined walls 20 which lead to a suitable fiber recovery means (not shown) below the exit 24 shown in FIG. 1. Those skilled in the art will be able to devise other suitable apparatus.

The method of this invention may be used for primary fractionation of pith and fiber fractions or in a secondary refining of fiber fractions recovered from an earlier treatment either wet" or dry"; for exampie, the fiber fraction recovered from the dry separation method disclosed in the above-identified application Ser. No. 743,344 now US. Pat. No. 3,537,142. In either event the resulting fiber fraction is substantially pith-free and, in addition, has relatively low solubles content. In actual trial runs it has been observed that the purity of this fiber fraction is so high that the caustic requirements in subsequent digestion to form paper pulp can be reduced as much as 5 to percent, as compared to previous practice with the substantially pure fiber fraction 95-97 percent fiber, soluble-free basis) recovered in accordance with my earlier application Ser. No. 743,344 now US. Pat. No. 3,537,142. For most efficient overall operation of a paper mill it will usually be preferred to practice the method of the present invention as a second stage final fractionation of fiber recovered after an initial rough fractionation, especially a dry" fractionation in accordance with the disclosure of my earlier application Ser. No. 743,344 now US. Pat. No. 3,537,142 as illustrated in the following specific example and schematically shown in the accompanying FIG. 2.

In FIG. 2 the method includes an initial dry fractionation in a primary depither 50 constructed and operated in accordance with my prior US. Pat. No. 3,537,142. The fibrous fraction is suitably recovered from the primary depither, as shown by arrow 50a, and conveyed in suitable manner by conveyor 51 to a secondary wet depithing in depither 52. The latter operation is performed as more specifically described herein and schematically illustrated in FIG. 1 hereof. The fibrous fraction from secondary depither 52 is again suitably recovered, as shown by arrow 52a, and conveyed, e.g., by conveyor 53, to the desired further processing. Thus, for example, the wet depithed fiber fraction may be settled in settling tank 54 to permit removal of excess aqueous carrier fluid through conduit 55. If desired, the solids from the settling tank may be withdrawn through conduit 56 and pressed in press 58 to remove still further aqueous carrier fluid through conduit 57. The final product is then recovered and carried by, e.g., conveyor 59 to pulping and papermaking operations. As explained below up to 50 or 55 percent of the sugar in an initial bagasse feed material can be recovered from the aqueous fluid removed from the depithing process via conduits 55 and/or 57. Where sugar recovery is the prime factor of concern a single stage wet depithing of the fresh bagasse in accordance with this invention is preferred. While the example illustrates the invention in connection with the processing of fresh bagasse (i.e., bagasse containing 48-52 percent moisture), it will be understood that it is also applicable to other mature stalk materials of a fibrous nature, such as bamboo, sorghum, corn stalks, broom stalks, broom straw, flax, hemp, sisal stalks, etc.

EXAMPLE Vertical Screening element: Height: 24 inches.

Inside Diameter:( upper end): 38 If! in.

(lower end):38 inches Openings: 1/4 or 3/16 inch total open surface area-40%) Height: approximately 24 inches Individual plate and hammer thickness: 1 /2 inch Hammer arrangement: 20 hammers total,

approximately one per every other plate, freely pivotable with angular displacement of from each hammer above or below.

Hammer twist angle: None [200 revolutions per minute.

Rotor Assembly:

Rotor Speed:

Three runs were conducted. In the first run Run A) fragments of fresh green bagasse (approximate moisture content 48-52 percent) was subjected to a dry depithing treatment in the above apparatus, in which the only carrier fluid was ambient air and all extraneous artificially created air pressure differences are precluded. In the second run (Run B) Run A was repeated except that water was fed concurrently to the feed opening with the bagasse fragments through a hose connection and in amounts sufficient to provide 5 pounds of water for each pound dry weight basis) of fiber in the bagasse feed. In the final run (Run C) Run B was repeated with the same weight ratio of water but using as feed preliminarily purified fibers previously recovered from the dry depithing treatment of Run A. Results were as follows:

Bagasse (fiber) Bagasse Fiber) Final Composition Feed original composition of Fiber Fraction Rate Tons (Dry Basis) Dry Basis) Per Hour, Bone-dry Thru On Thru On Run Basis 14 Mesh 14 Mesh 14 Mesh 14 Mesh A 8.5 36-38% 62-64% 20-22% I 78-80% B 8.5 36-38% 62-64% 20-22% 78-80% C 8.5 20-2 2% 78-80% 18-20% 80-82% Solubles in Fiber Fractions from Run A B C Solvent Weight Percent Soluble Cold Water 5.2 3.3 3.3 Hot Water 6.0 4.0 4.0 Caustic Soda( 1%) 28.6 25.8 25.8 Alcohol/Benzene 4.2 3.7 3.7

In subsequent pulping operations it has been observed that the caustic requirements for the fiber fraction from Run A are about 110 kilograms per ton of fiber. For the fiber produced in accordance with Runs B and C the requirements are about 100 kilograms and about -98 kilograms, respectively, per ton of fiber pulped.

Pulps prepared from the fiber fractions of Run A and Run C show the following comparative properties.

Pulp prepared from fiber of Property Run A Run C Freeness (cubic centimeters) 828 838 permanganate Number 20 19.9 BeatingsFreeness 650 Average Tear Factor 66.6 67.9

Average Tensile 4409 4870 Average Burst Factor 27.8 28.6 Beatings-Freeness 500 Average Tear Factor 59.3 60.3

Average Tensile 5059 5490 Average Burst Factor 32.0 35.2

In the practice of the wet depithing process of this invention I have found that it is not necessary to partially twist the initial hammers of the rotor assembly in order to obtain adequate feed rates. Thus in the practice of the present invention all hammers will typically be horizontal or flat) throughout their entire length. I have further found that power requirements for the wet depithing process of this invention can be reduced by twisting some or all of the hammers 90, in effect providing vertical hammers. In one arrangement providing excellent results I used 12 hammers in all one for every four plates in the rotor assembly) in which the first two were horizontal and all of the remaining were vertical, i.e., twisted 90. Such arrangements are not satisfactory in practicing the dry depithing method of my earlier application Ser. No. 743,344.

in practicing the process of this invention, using fresh water as the aqueous carrier liquid, I have been able to recover up to about 25 percent of the sugar in the original bagasse when the invention is applied as a secondary wet depithing of the fiber fraction from an initial dry depithing in accordance with my above-mentioned earlier patent application. This can be increased to a recovery of about 33-34 percent, based on the sugar content of the original bagasse, if the wet depithed fiber fraction is pressed to return its moisture content to its original value of about 50 percent. When using the wet depithing process of this invention as the initial depithing stage, followed by compressing the recovered fiber fraction to return its moisture content to 50 percent, the sugar recovery can be increased to approximately 50 to 55 percent of the sugar retained in the original bagasse.

What is claimed is 1. Method fro processing crushed fibrous stalks containing pith, to separate the pith and the fiber, comprising gravity feeding fragments of the crushed fibrous stalks into the upper end of a vertically disposed screening element surrounded by a closed chamber and having a substantially circular cross section; simultaneously feeding to the upper end of said screening element sufficient amounts of an aqueous carrier liquid non-reactive with the fiber component to provide at least about 4.5 parts by weight of said liquid for each part by weight (dry weight basis) of the said fiber component; imparting a helical flow path to the mixture of carrier liquid and said fragments and centrifugally forcing the fragments against the inner surface of the screening element by means of rotating agitating members, while precluding any extraneous artificially created air pressure differences, to thereby form a layer of the fragments on the interior surface of the screening element, substantially all of which are oriented with their axes coaxial with the screening element axis; continuing to feed the said fragments toward the lower end of the screening element by continuing contact with the rotating agitating members and because of the forces of gravity acting thereupon while separating the pith therefrom by rolling and rubbing action of the fragments on each other; gradually increasing the working on said fragments as they progress from the upper inlet end to the lower exit end of the screening element to separate the pith particles from the fiber particles and permit the pith particles to absorb substantial carrier liquid; forcing the separated, now much heavier pith particles to the exterior of the screening element by the centrifugal forces on the helically moving mass; and separately recovering substantially pith-free fibers from the lower exit end of the screening element.

2. Method as defined in claim 1 wherein said carrier liq iid is water.

Method as defined in claim 2 wherein the water to dry fiber ratio is from about 5: l to about 10:1.

4. Method as defined in claim 1 wherein the fragments of crushed fibrous stalks are first subjected to an initial dry separation treatment comprising gravity feeding fragments of the crushed fibrous stalks into the upper end of a vertically disposed cylindrical screening element surrounded by a closed chamber and having a substantially circular cross section tapering inwardly toward the lower end of said screening element; imparting a helical flow path to the said fragments and centrifugally forcing them against the inner surface of the said screening element by means of rotating agitating members, while precluding any extraneous artificially created air pressure differences, to thereby form a layer of the fragments on the interior surface of the screening element, substantially all of which are oriented with their axes coaxial with the screening element axis; continuing to feed the said fragments toward the lower end of the screening element by continuing contact with the rotating agitating members and because of the forces of gravity acting thereupon while separating the pith therefrom by the rolling and rubbing action of the fragments on each other; gradually increasing the working on said fragments as they progress from the upper inlet end to the lower exit end of the tapered screening element forcing the separated pith particles to the exterior of the screening element by the centrifugal forces on the helically moving mass; and separately recovering substantially pithfree fibers containing about 'to about 97 percent fiber( dry, soluble-free basis) from the lower exit end of the screening element; whereafter the substantially pith-free fibers are still further depithed by processing with simultaneously fed aqueous carrier liquid in accordance with the method of claim 1, whereby the final separately recovered pith-free fibers contain at least about 98 weight percent fiber (dry, soluble-free basis). 

1. Method fro processing crushed fibrous stalks containing pith, to separate the pith and the fiber, comprising gravity feeding fragments of the crushed fibrous stalks into the upper end of a vertically disposed screening element surrounded by a closed chamber and having a substantially circular cross section; simultaneously feeding to the upper end of said screening element sufficient amounts of an aqueous carrier liquid non-reactive with the fiber component to provide at least about 4.5 parts by weight of said liquid for each part by weight (dry weight basis) of the said fiber component; imparting a helical flow path to the mixture of carrier liquid and said fragments and centrifugally forcing the fragments against the inner surface of the screening element by means of rotating agitating members, while precluding any extraneous artificially created air pressure differences, to thereby form a layer of the fragments on the interior surface of the screening element, substantially all of which are oriented with their axes coaxial with the screening element axis; continuing to feed the said fragments toward the lower end of the screening element by continuing contact with the rotating agitating members and because of the forces of gravity acting thereupon while separating the pith therefrom by rolling and rubbing action of the fragments on each other; gradually increasing the working on said fragments as they progress from the upper inlet end to the lower exit end of the screening element to separate the pith particles from the fiber particles and permit the pith particles to absorb substantial carrier liquid; forcing the separated, now much heavier pith particles to the exterior of the screening element by the centrifugal forces on the helically moving mass; and separately recovering substantially pith-free fibers from the lower exit end of the screening element.
 2. Method as defined in claim 1 wherein said carrier liquid is water.
 3. Method as defined in claim 2 wherein the water to dry fiber ratio is from about 5:1 to about 10:1.
 4. Method as defined in claim 1 wherein the fragments of crushed fibrous stalks are first subjected to an initial ''''dry'''' separation treatment comprising gravity feeding fragments of the crushed fibrous stalks into the upper end of a vertically disposed cylindrical screening element surrounded by a closed chamber and having a substantially circular cross section tapering inwardly toward the lower end of said screening element; imparting a helical flow path to the said fragments and centrifugally forcing them against the inner surface of the said screening element by means of rotating agitating members, while precluding any extraneous artificially created air pressure differences, to thereby form a layer of the fragments on the interior surface of the screening element, substantially all of which are oriented with their axes coaxial with the screening element axis; continuing to feed the said fragments toward the lower end of the screening element by continuing contact with the rotating agitating members and because of the forces of gravity acting thereupon while separating the pith therefrom by the rolling and rubbing action of the fragments on each other; gradually incReasing the working on said fragments as they progress from the upper inlet end to the lower exit end of the tapered screening element forcing the separated pith particles to the exterior of the screening element by the centrifugal forces on the helically moving mass; and separately recovering substantially pith-free fibers containing about 95 to about 97 percent fiber (dry, soluble-free basis) from the lower exit end of the screening element; whereafter the substantially pith-free fibers are still further depithed by processing with simultaneously fed aqueous carrier liquid in accordance with the method of claim 1, whereby the final separately recovered pith-free fibers contain at least about 98 weight percent fiber (dry, soluble-free basis). 